Re: Using POPCNT and other advanced bit manipulation instructions

From: Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>
To: Kyotaro HORIGUCHI <horiguchi(dot)kyotaro(at)lab(dot)ntt(dot)co(dot)jp>
Cc: tgl(at)sss(dot)pgh(dot)pa(dot)us, andres(at)anarazel(dot)de, thomas(dot)munro(at)enterprisedb(dot)com, andrew(at)tao11(dot)riddles(dot)org(dot)uk, david(dot)rowley(at)2ndquadrant(dot)com, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: Using POPCNT and other advanced bit manipulation instructions
Date: 2019-02-15 14:41:23
Message-ID: 20190215144123.GA31671@alvherre.pgsql
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On 2019-Feb-15, Alvaro Herrera wrote:

> On 2019-Feb-15, Kyotaro HORIGUCHI wrote:

> > And even worse lzcntx is accidentially "fallback"s to bsrx on
> > unsupported CPUs, which leads to bogus results.
> > __builtin_clzll(8) = 3, which should be 60.
>
> I'm not sure I understand this point. Are you saying that if we use
> -mlzcnt to compile, then the compiler builtin is broken in platforms
> that don't support the lzcnt instruction? That's horrible. Let's stay
> away from -mlzcnt then.

Ah, I understand it now:
https://stackoverflow.com/questions/25683690/confusion-about-bsr-and-lzcnt/43443701#43443701
if you call LZCNT/TZCNT on a CPU that doesn't support it, it won't raise
SIGILL or anything ... it'll just silently compute the wrong result.
That's certainly not what I call a fallback!

I think David's code was correct because it was testing CPUID for
instruction support before using the specialized code (well, except that
he forgot to add the right compiler option to *enable* the LZCNT/TZCNT
instructions in the first place); but given subsequent discussion that
the instruction is not worth it anyway, we might as well ignore it.

--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services

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